1. Field of the Invention
The present invention relates to a transparent touch panel used for operation of various kinds of electronic equipment.
2. Background Art
In recent years, electronic equipment has had more advanced and diversified functions. Accordingly, there is an increasing number of such equipment in which characters, marks, and icons displayed on display elements of liquid crystal are recognized and selected through a transparent touch panel attached in front of the display elements so that individual functions of the equipment are switched.
Such a conventional touch panel is described with reference to FIGS. 5 and 6. To facilitate understanding of a structure thereof, the dimension in the direction of the thickness is enlarged in the drawings.
FIG. 5 is a sectional view of a conventional transparent touch panel. On the bottom surface of optically transparent upper substrate 21 made of polyethylene terephthalate film, polycarbonate film, or the like, optically transparent first conductive layer 22 made of indium tin oxide, tin oxide, or other materials, is formed by vacuum sputtering or other methods. On the other hand, on the top surface of optically transparent lower substrate 23 made of polyethylene terephthalate film, glass, acryl or the like, optically transparent lower conductive layer 24 like first conductive layer 22 is formed. On the top surface of lower conductive layer 24, a plurality of dot spacers 25 made of an insulating resin, e.g. epoxy and silicon, are formed at regular intervals to hold a predetermined space between first conductive layer 22 and lower conductive layer 24.
A pair of upper electrodes 26 extends from both edges of first conductive layer 22. A pair of lower electrodes 27 extends from both edges of lower conductive layer 24 in a direction orthogonal to upper electrodes 26. Each electrode is made by printing paste made of silver, carbon, or other materials. The ends of each lower electrode 27 extend to the edges of lower substrate 23. Each of upper electrodes 26 is wired to the top surface of lower substrate 23 via a through-hole (not shown) filled with conductive agent, and the ends of each upper electrode 26 extend to the edges of lower substrate 23 in parallel with lower electrode 27.
Moreover, the outer peripheries of upper substrate 21 and lower substrate 23 are bonded by a frame-shaped spacer (not shown) having an adhesive agent applied to top and bottom surfaces thereof so that first conductive layer 22 is opposed to lower conductive layer 24 with a predetermined space held therebetween. The edges of upper substrate 21 and lower substrate 23 sandwich wiring board 29 that is made of polyethylene terephthalate film, polycarbonate film, or the like and has a plurality of wiring patterns 28 made of copper or other materials formed on the bottom surface.
At the ends of wiring patterns 28, joints 28A plated with nickel, gold, or other materials are formed. Filled between joints 28A and upper electrode 26 or lower electrode 27 is anisotropic conductive adhesive (hereinafter referred to as “adhesive”) 30 containing conductive grains 30B made of carbon dispersed in synthetic resin 30A, e.g. polyester and chloroprene rubber. In other words, as shown in a partially sectional view of FIG. 6, joints 28A at the ends of wiring patterns 28 and upper electrode 26 or lower electrode 27 are bonded by synthetic resin 30A in adhesive 30. They are also electrically connected by the top and bottom surfaces of conductive grains 30B.
After wiring board 29 is sandwiched between upper substrate 21 and lower substrate 23, joints 28A and upper electrode 27 or lower electrode 28 are bonded by hot-pressing adhesive 30 applied to the side of lower substrate 23 or wiring board 29. As a result, as shown in FIG. 6, the top surfaces of conductive grains 30B are pressed by joints 28A plated with a relatively hard metal, such as nickel and gold, and the bottom surfaces are substantially embedded into relatively soft upper electrode 26 or lower electrode layer 27.
In this structure, wiring patterns 28 on wiring board 29 are coupled to a detecting circuit (not shown) of electronic equipment via a connector or by soldering. Depressing the top surface of upper substrate 21 with a finger, pen, or the like flexes upper substrate 21 and brings first conductive layer 22 in the depressed position into contact with lower conductive layer 24. Then, the detecting circuit sequentially applies voltages to two pairs of upper electrode 26 and lower electrode 27. According to the resistance ratio of these two pairs of electrodes, the detecting circuit detects the depressed position. A transparent touch panel structured to sandwich a wiring board between two transparent substrates in this manner is disclosed in Japanese Patent Application Unexamined Publication No. H04-12421, for example.
Because the bottom surfaces of conductive grains 30B are substantially embedded into upper electrode 26 or lower electrode 27, wiring board 29 and upper substrate 21 or lower substrate 23 are connected in a stable manner. However, the top surfaces of conductive grains 30B are only in contact with joints 28A. For this reason, when a transparent touch panel is used at high ambient temperatures and synthetic resin 30A softens, the connection of this portion is not ensured. Thus, electrical connection between wiring pattern 28 and upper electrode 26 or lower electrode 27 is likely to be unstable.